Dave our own pond is the same size as yours we dont have the waterfall relying on a Spindrifter bottom drain cover to supply oxygen which creates a downward flow around the pond .
Koi pox isnt a problem its seasonal and disapates as the water warms up again in spring so I wouldnt worry about it, yes I know it looks ugly but it does go...
Even goldfish suffer from it but with them its known as fish pox its the same virus.
I believe once it shows on a fish its acctually the end stage of the virus a little like when we humans get chicken pox,in saying this however weve noticed over the years looking at friends ponds that some koi get covered in it, others just a little, some dont even come down with it at all
Your main worry keeping koi would be the introduction of KHV (Koi Herpes Virus)
which kills at about an 80% mortality with survivors either being ruthanized or placed in Permanent QT with seperate filters nets and equipment .
Goldfish suffer GHV (Goldfish Herpes Virus),a varient of KHV (not sure which came first), it could be the other way around but it has less of a mortality rate however the same permanent QT proceedures must be practiced but the odds on you getting these two viruses are minimal.
We over winter using Polycarbonate sheeting covering the pond, have everything from the filter housing and pond insulated by 22mm polystrene sheets with further insulation around the pond of silversided bubble wrap and recycled plastic loft insulation, and have a small 800 watt Oil heated radiator which cuts off at temperature leaving the oil to cool then cuts back in again.
It works and keeps our pond at around 13-14c throughout the winter dipping to about 11.c when exeptionally cold (our UK temeratures being less than the US).
I do however have a friend in Canada who I got to do the same as your good self in that she brings her koi and goldfish indoors to her husbands workshop and is no longer looing koi through the intense cold she suffers every year -24c with a windchill of -54c.
As to a good test kit m'mmm its a toss up between API pr Tetra ( weve used both but prefare Tetra over API).
Incidentally I used to keep our koi indoors in a huge aquarium but ill health forced the move outside due to my inability to do maintenance issues anymore thanx to chronic Osteo Arthritis, working with slide valves and a hose takes far less time and doesnt hurt the knees as much , this will be our 27th year as koi keepers.
rgrds
Dave